a new version of the reference software for JPEG XT is avaiable at http://www.jpeg.org/jpegxt/software.html for download. This new software matches the latest "final draft" version of the specs of XT, and also includes a couple of new options and features to improve the coding performance. Most notably, you can select the quantization matrix and enable an optimized deadzone quantizer.
A further enhanced experimental version of the same software is available for testing at the jpeg online test facility at http://jpegonline.rus.uni-stuttgart.de/ . The version presented there adds some features from mozjpeg, most notably the trellis quantizer and a (completely different, though) encoder-side deblocking filter.
I also added three new images, compoound1, compound2 and otto2 which are good tests for the deringing filter. These images are mixed text/graphics content and are hence a typical victim of the Gibbs rining phenomenon which is filtered away by the new option in the experimental software.
Have fun testing!